Ability to enable or disable nav monitor

In order to handle the message based concurrency model used by Artoo (due to using Celluloid), we do not want to have callbacks to actors taking place within a mutex. Unfortunately, this is how the current Argus nav callbacks are implemented.

The simplest thing is probably just to allow the NavMonitor to be enabled or disabled by consumers of the Argus gem. This way both cases can be easily accommodated, which is what this pull request addresses.